Stone, or granular, columns are formed in the ground to provide a foundation (increase the load bearing properties of the ground) and/or improve drainage.
There are a variety of methods for creating granular columns, in one method the hole is drilled (a poker can be inserted and withdrawn in some instances) and the drill extracted, the hole is part filled with stone aggregate (10 mm to around 75 mm). A cone shaped vibrator is then used to compact this fill as more is added until the column is formed. As a hole needs to be drilled first this method is not suitable in collapsing ground; the walls of the hole collapse which may prevent the column from being formed or the column can be contaminated during construction. In addition any fine material may be blown out creating a spoil problem. The method does not work with high ground water levels.
To overcome some of the weaknesses of the above mentioned method it is possible to feed the stone to the underside of the vibrator and withdraw the vibrator as the stone fills the void. The fines blowout can still be a problem but this can be reduced by using compressed air. The column can still be contaminated with ground material and some ground types can be hard to penetrate. One significant drawback to this method is the equipment cost.
To improve the quality control of the granular column a number of solutions have been proposed. For example U.S. Pat. No. 5,279,502 describes a hollow tube which includes an impellor at the tip. In use the tube penetrates the ground and, when at the desired depth crushed stone (aggregate) is fed through the centre of the tube and expelled through the impellor. The impellor is designed to distribute the crushed stone fed through the hollow tube out radially and as such is not suitable for ‘drilling the hole’. In use the impellor forces the stone out radially and forms an essentially radially compacted granular column as it is withdrawn. The author indicates that by applying the compaction forces in this way the granular column can be formed faster and with more predicable properties. The method is directed to applications with soft cohesive soils though a variation can allow it to be used with harder soils. The method is not really suitable for ground which includes high levels of non-cohesive materials, such as sand.
Any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field.
The present invention provides a machine and/or method for forming a granular column that is intended to overcome one or more of the deficiencies of the methods mentioned above, or provide the consumer with a useful choice.
The present invention intends to provide a machine and/or method that forms in ground granular columns without the need for vibratory compaction, and primarily (greater than about 66%) vertical compaction.